Although laser technology has made remarkable progress at present, laser oscillation is not implemented over the entire wavelength domain. Accordingly, wavelength conversion technology utilizing nonlinear optical effect is an important technology to obtain coherent light in a wavelength range in which the laser oscillation is not easy.
Among nonlinear optical effect, the application of wavelength conversion devices is expected which generate a second harmonic, sum frequency or difference frequency by pseudo-phase matching by making use of the second order nonlinear optical effect (see Patent Document 1).
FIG. 1 shows a configuration of a conventional pseudo-phase matching type wavelength conversion device. A multiplexer 11 multiplexes pumping light A (wavelength λ1) from a semiconductor laser light source and pumping light B (wavelength λ2) from another semiconductor laser light source, and launches into a nonlinear waveguide 12 having a polarization inversion structure. The waveguide 12 converts the pumping light A to difference frequency light C with a wavelength λ3 and emits it together with the pumping light B. A demultiplexer 13 separates the difference frequency light C from the pumping light B.
For example, assume that the pumping light A has a wavelength λ1=1.06 μm, and that the pumping light B with a wavelength λ2=1.55 μm is input. In this case, the wavelength converted light C with a wavelength λ3=3.35 μm can be obtained by the difference-frequency generation.
Making use of such a wavelength conversion device as a middle infrared laser light source can implement highly sensitive gas sensors and the like utilizing the middle infrared light.
According to another embodiment in FIG. 1, the multiplexer 11 multiplexes the comparatively high intensity pumping light A with the pumping light B, and launches into the nonlinear waveguide 12 having the polarization inversion structure. The waveguide 12 generates the wavelength converted light C of the pumping light A and the pumping light B, and emits it. For example, assume that the pumping light A has a wavelength λ1=1.06 μm and the pumping light B has a wavelength λ2=1.32 μm. Then, the wavelength converted light C with a wavelength λ3=0.59 μm, which is yellow visible light, is obtained by the sum-frequency generation.
A yellow visible light source using the wavelength conversion device based on the sum-frequency generation is applicable as a light source for measuring refractive indices instead of a conventional D line light source of an Na lamp. In addition, the yellow visible light source using the wavelength conversion device based on the sum-frequency generation has marked effect on increasing the sensitivity of optical equipment using visible light such as fluorescence microscopes.
Such a yellow visible light source includes in its casing a 1.06 μm semiconductor laser that stabilizes its wavelength using an external resonator utilizing a fiber Bragg grating (FBG) as the pumping light, a DFB (Distributed Feedback) laser whose oscillation wavelength is 1.32 μm, a multiplexing means such as a WDM coupler, and a modularized wavelength conversion device. Here, as the light source used as the pumping light, a light source with single mode oscillation such as a DBR (Distributed Bragg Reflector) laser and DFB laser is preferable. When it does not have the single mode oscillation, it is preferable that a light source be used whose wavelength is stabilized by adding an external resonator using an FBG.
The FBG, which has a Bragg diffraction grating formed in a core section of an optical fiber, is an optical fiber type device with a characteristic of reflecting only light with a particular wavelength. The FBG has as its property a low loss, good coupling characteristic with an optical fiber, and superior reflection characteristics. Thus, besides the reflection light filter, the FBG is widely applied to wavelength control devices, optical sensor devices and dispersion compensation devices.
The DFB laser is a semiconductor laser that oscillates laser light by confining light to an active region by operating a periodic shape built in a laser chip as a diffraction grating, and by reflecting only light with a particular wavelength. It is superior to a Fabry-Perot semiconductor laser without having a diffraction grating in the monochromaticity of the wavelength of the laser light, and is suitable for light signal transmission beyond several kilometers.
FIG. 2 illustrates pseudo-phase matching conditions for obtaining green light with a wavelength of 0.53 μm by the second-harmonic generation. FIG. 2 is a diagram of the pseudo-phase matching curve calculated under the assumption that lithium niobate is used as a nonlinear optical material, the polarization inversion period is 6.76 μm and a wavelength conversion device with a length of 10 mm is used. The horizontal axis shows the wavelength of the pumping light, and the vertical axis shows the normalized light intensity of the second harmonic obtained. FIG. 2 shows that the pseudo-phase matching band is equal to or less than 0.2 nm. Accordingly, the oscillation wavelength of a 1.06 μm semiconductor laser must be stabilized within the spectral width of 0.2 nm. To obtain the light output of stable wavelength converted light, the wavelength stabilization is an essential condition because the pseudo-phase matching bandwidth of the wavelength conversion device used for obtaining the visible light is narrow.
The wavelength converted light C generated under such conditions inherits a coherent characteristic from the semiconductor laser serving as the pumping light, and is effective as the visible light source for increasing sensitivity of refractive index measurement and of fluorescent protein observation with a fluorescence microscope. To increase the measurement sensitivity, however, a high extinction factor is required, and an ON/OFF modulation function must be provided.
Until now, however, the ON/OFF modulation is carried out using a single semiconductor laser as the visible light source, or by connecting an external AO modulator to a solid laser. Thus, the ON/OFF modulation function has not yet been implemented in the light source that generates the difference frequency or sum frequency by using the optical waveguide composed of a nonlinear optical material and two semiconductor laser light sources.
FIG. 3 illustrates current-light output characteristics of a 1.06 μm-band semiconductor laser connected to an external FBG. The semiconductor laser connected to the FBG usually has a reflection band of about 2 nm due to the FBG, and is placed in a multimode state in which a plurality of wavelengths oscillate within that range. Although a result obtained by differentiating the light output by current is referred to as differential efficiency, the differential efficiency characteristics represented by broken lines have a plurality of discontinuous locations, and hence the current-light output characteristics have minute discontinuities. In the case where such discontinuities are present, it is very difficult to carry out the light output stabilization control (APC) for the semiconductor laser. Thus, generally, the external resonator type semiconductor laser having the external FBG has not been used in a modulated state.
FIG. 11A illustrates current-light output characteristics of a 1.32 μm-band DFB laser. Controlling the current of the 1.32 μm-band DFB laser enables the output of the wavelength converted light to be turn on and off in accordance with the current-light output characteristics of FIG. 11A, and enables the light source apparatus with modulation function to have a conversion function.
The 1.32 μm-band DFB laser employed here has Ith=10 mA or so, and operates stably at the signal wavelength even when the operation current is placed at about 30 times the Ith to increase the light output. FIG. 11A shows the differential efficiency by broken lines. Except for the threshold value, no discontinuous locations appear in the differential efficiency characteristics, and no module jumps of the oscillation wavelength occur. However, increasing the operation current of the 1.32 μm-band DFB laser up to 300 mA brings about a temperature rise in the device, which shifts the oscillation wavelength about 0.8 nm with keeping the single wavelength. The range of variation of 0.8 nm of the oscillation wavelength is four times wider than the foregoing pseudo-phase matching band. In addition, the portion outside the pseudo-phase matching band does not contribute to the wavelength converted light. Accordingly, it is impossible to increase the operation current beyond about ¼ of 300 mA, or about 80 mA, substantially, and hence the light output obtained reduces to about ¼. This means that the output intensity of the wavelength converted light also reduces to about ¼. Thus, the practical output intensity of the wavelength converted light cannot be achieved by only carrying out the modulation in the 1.32 μm-band DFB laser.
Patent Document 1: Japanese Patent Laid-open No. 2003-140214.